In the prior art, material coatings typically have been applied to metallic or other surfaces for protective or restorative purposes, such as wear and corrosion resistance, thermal barrier coatings (“TBCs”), protection against oxidation, and dimensional restoration. Coatings made from ceramic materials in particular offer resistance against abrasive wear and thermal shock and provide electrical insulation.
The thermal spray coating process has evolved from a method for patching or repairing material surfaces to a technique enabling surface microstructure design and enhancement. As is known to those skilled in the art, thermal spraying involves any of several methods by which a coating material is placed in the path of a spray jet, heated until the material softens or melts, and propelled to the surface of a prepared substrate to form a deposit. U.S. Pat. No. 5,900,283 to Vakil et al. and U.S. Pat. No. 5,985,368 to Sangeeta et al. describe various methods of depositing a protective coating on a metal-based substrate, including thermal spraying.
Thermal spraying processes include flame spraying and plasma spraying. In each process, the material is impacted as it is deposited on the substrate and is allowed to cool and solidify to form a coating on the substrate. The deformation of the softened or molten material upon impact with the substrate, and, in some cases, the force of the impact, is sufficient for the material to mechanically bond to the surface. In thermal spraying processes, the particles sprayed, upon impact with the substrate surface mechanically lock and/or bond into the profile of the surface.
Initially, thermal spray technology focused on techniques of heating particles into a plastic state before impact to effect bonding with the surface. Methods of high velocity spraying now exist whereby extreme spray velocities are generated. Powder material is injected into a focused gas stream and imparted with enough velocity that the force of impact of particles with the substrate is sufficient to achieve adequate bonding.
Flame spraying and plasma spraying are conventional methods by which coatings are applied. Flame spraying involves the heating and projecting of the coating material through the use of an oxygen fuel flame and pressurized carrier gas jet. Coating material is melted, atomized, or softened as it is fed into the flame, and the soft or molten particulate is ejected in a directed stream through the nozzle of the spray gun toward the substrate. Like flame spraying, plasma spraying also heats and projects particles of a material toward a work surface or substrate. A plasma gun replaces the flame used in older spray systems with a stream of highly ionized inert gas, plasma, and imparts greater velocity to the powder. In the plasma spray system, an electric arc is created and a mixture of combustible gases is ignited to create a high temperature flame. The resulting plasma flame can be pushed forward in front of the gun. When powder is injected into the plasma flame near the front of the plasma spray gun nozzle, the gases expand rapidly, and the resulting velocity of the heated powder particulate propels it to the substrate. In both flame spraying and plasma spraying, large size particles or powders (typically >10 micrometers) are injected.
Coatings for protective and similar applications are formed by spraying hot powder particles onto a substrate to form coatings with thicknesses ranging from about ten up to hundreds of micrometers. The resulting coating exhibits a relatively dense and smooth, or low-profile, surface with large particle sizes. Thermally sprayed ceramic coatings may exhibit physical characteristics such as a durable, higher profile surface suitable for gripping and anti-skid applications. In applications such as sliding wear situations, in which a smoother surface is required, the coated surfaces may be further refined by grinding and polishing.
Thermally sprayed coatings have a relatively weak bond with the substrate. Internal stresses, especially in thicker coatings, and high stress wear patterns may exceed the bond between the coating and substrate and cause failure. Surface treatments that are known in the art, such as grit blasting, enhance the work surface and provide an anchor profile for the coating so that it will better adhere to the substrate. The resulting high profile coating can be polished or otherwise refined as discussed above to achieve a smooth surface in, for example, protective coating applications.
In the present invention, the thermal spraying process produces on a surface a small particle size and high surface area coating that is desirable in applications where the coating is a catalyst itself, or where the treated surface is a precursor surface for a further coating. The thermal sprayed treated substrate surface requires no further refinement.
A method for applying and forming a thermally sprayed coating on a substrate that yields a thin layer of a catalyst coating with small particle size and high surface area is the desirable object achieved by the present invention. A coating applied to a substrate using thermal spraying techniques in which the coating has mechanical stability and promotes catalytic reactions is yet another object achieved by the present invention
Catalytic material has been used as a coating on ceramic or metallic substrates in applications such as, for example, catalytic converters in automobile exhaust systems to reduce the emission of noxious gases. In the prior art, the substrate is typically coated with a catalyst by immersion in a slurry containing the catalytic material. Problems arise in that catalytic material does not adhere as well to a metallic substrate as it does to a ceramic substrate and the coating is not uniformly distributed.
U.S. Pat. No. 5,721,188 to Sung et al. discloses a thermal spray method for adhering a catalytic material to a metal substrate. The disclosed method involves thermally spraying refractory oxide particles onto a substrate for the primary purpose of attaining an undercoat having high surface roughness, and the subsequent application of a separate catalytic material to the undercoated substrate. Refractory oxide powders ranging in average particle size from 13 to 180 microns are thermally sprayed onto the substrate (See, for example, column 3, lines 27-32). Catalytic material is then applied to the undercoated substrate by immersing the substrate in a wash coat slurry containing the catalytic material. The coated substrate is then dried and calcined (Id., column 4, lines 30-35). Sung et al. recognized the need for a method to improve the adhesion between metallic substrates and catalytic materials disposed thereon (Ibid., column 1, lines 29-31) but do not propose a method for adhering a catalytic material directly to a metallic substrate.